The present invention relates to amorphous silicon solar cells.
Solar cells are photovoltaic devices which are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as a result of what is well-known in the solar cell field as the "photovoltaic effect". When solar radiation impinges on a solar cell, it is absorbed by the active region of the cell, causing electrons and holes to be generated. The electrons and holes are separated by the electric field resulting from the PIN junction in the solar cell. The electric field is inherent in a semiconductor layer adjacent regions of P type, intrinsic, and N type hydrogenated amorphous silicon. Absorption of solar radiation of the appropriate wavelength produces electron-hole pairs in the intrinisic region of the semiconductor layer. The separation of the electron-hole pairs, with the electrons flowing toward the region of N type conductivity, and the holes flowing toward the region of P type conductivity gives rise to the photovoltage and photocurrent of the cell. The overall performance of the solar cell is maximized by increasing the total number of photons of differing energy which are absorbed by the semiconductor device.
I invented and disclosed in U.S. Pat. No. 4,272,641, issued June 9, 1981, and entitled "Tandem Junction Amorphous Silicon Solar Cells," incorporated herein by reference, a tandem-junction structure for an improved amorphous silicon solar cell. The structure comprises two or more layers of hydrogenated amorphous silicon arranged in a tandem, stacked configuration with one optical path which are electrically interconnected by a tunnel junction. The layers of hydrogenated amorphous silicon include regions of differing conductivity which provide a built-in electric field in each semiconductor layer. The layers can have the same or, in a preferred embodiment, differing bandgaps to absorb more completely the distribution of photons of different energies in the solar spectrum. Thus, my solar cell structure exhibits increased performance through absorption of a greater portion of the solar spectrum. However, a grid electrode is required in large-area solar cells to collect the photocurrent. The grid electrode can shield up to about 10 percent of the active region of the solar cell from the available solar radiation. In addition, as the solar cell area and the current from the solar cell increase, the complexity of the grid electrode also increases. As a result there is a practical limitation on the area of a solar cell.
Thus, it would be highly desirable to have a structure which could maximize the absorption of solar radiation without the shielding effect and area limitations of the grid electrode.